The Study On Simplified Model Of Distillation Plate With Shortcut Gas

Distillation is the most widely used separation process in chemical and petrochemical industry.A distillation column constructs a vast 3-dimension space where complex mass transfer,heat exchange and hydraulic phenomena occur among distributed vapor and liquid flows and accumulations.The design and operation of distillation column require rich engineering experience,and the modeling and simulation of distillation column is always an important and challenging research topic.By dividing the incoming gas from the lower plate into the immerging part and the shortcut part,a simple model for distillation plates is proposed where the deviation from an equilibrium stage is accounted for with a shortcut gas calculated theoretically.For the bubble plate,shortcut gas fraction is derived from the Stefan-Maxwell equation,and directly related to the operation conditions of the plate and the physical properties of the gas and liquid phases.As a derivative of the model proposed,the Murphree efficiency is explained as fraction of the evaporated vapor over the leaving gas in total.This novel physical meaning extends naturally the Mur:phree efficiency to plates working with multicomponent fluids,to equilibrium stages such as reboilers as commonly assumed,and to unordinary plates where vaporization and condensation are changed significantly by external effects such as heat removal and stripping.For a plate of given geometry and substance system,the proposed model is predictive in the sense that its only tuning parameter can be considered as a constant within a range of operating conditions.By simulating seven distillation columns of cyclohexane/n-heptane column,ethanol/water column and so on,the powerful correlating and predicting capabilities of the proposed model for ordinary and unordinary plates working with binary and multicomponent fluids are testified against three well-established efficiency models and the simulation results of RadFrac module with Rate-Based calculation type in Aspen Plus.Under the same steady-state condition,the overall efficiency calculated from the running data of this model is consistent with that of the actual production data.With this split or rearrangement of gas flow,the actual trays,including the plate with heat-exchange or stripping gas,can be uniformly approximated to the improved equilibrium stage model in this paper.